This question has been coming up a lot recently – some ladies at the knit night, my own hat adventures, and even in email, so I thought I would post the formula that I use for it.
Step 1 – Multiply the number of stitches you need to decrease by 2 - because each decrease uses up two stitches. If your decrease uses 3 stitches (k3tog, or slip1, k2tog, psso for example) then you multiply by three, and so forth.
Step 2 – Subtract that number from your total number of stitches. This gives you the number of stitches that your decreases will need to be space over; ie, all the stitches that won’t be used in the decreases.
Step 3 - Divide the number of stitches not used in your decreases by the number of stitches you need to decrease.
Step 4 — if it is really important to place the first and last decreases equidistant from the ends of the row (if something is being worked in the round, or will be seamed round), divide the step 3 number by two — this tells you how many stitches to knit from the beginning of the round before the first increase.
Here is is written as formula -
Step 1 – total stitches – (decreases x number of stitches used in decrease, usually 2) = stitches not used in decreasing — this is steps 1 and 2 from above.
Step 2 – stitches not used in decreasing / decreases = stitches to knit between decreases
Step 3 – stitches knit between decreases / 2 = stitches to knit before first decrease from beginning of row.
And here is a sample:
If I have 54 stitches, and I need to decrease 3 evenly over one round, first I multiply 3×2 — which gives me 6. (Step 1)
54 minus 6 gives me 48. This is how many stitches that will be left after the decreases. (Step 2)
Then I divide 48 by 3 — which gives me 16, the number of stitches to knit between each decrease. (Step 3)
I’m working in the round though, and want to space evenly from the beginning of my row — so I divide 16 by 2 — which gives me 8. (Step 4)
To knit the round, I’ll k8, k2tog, k 16, k2tog, k16, k2tog, k8 – leaving 51 total stitches. A successful decrease of three, evenly distributed around.
The main problem you will run into is fractional numbers , which in knitting terms means extra stitches. Just fudge it — slide the extra stitches in as evenly as possible, maybe on opposite sides of your piece, or if the pattern allows it, maybe work a k3tog instead of a k2tog to even them out.
Lagniappe: You can also use step two of the formula to space increases evenly. It varies slightly for increases worked in between stitches (YO, lifted bar increases) and increases worked in a stitch (Kfb is the only one I can recall right now)
Those formulas written independently are
For increases between stitches: total stitches / number of increases = stitches to work between each increase,
and for increases in stitches: (total stitches-number of increases) / number of increases = stitches to work between each increase.